serial port for an alarm system

[mfino]

I have an alarm system set to receive inputs from LabVIEW-controlled digital I/O cards. All the digital ports are fully utilized for data input. When an alarm state is reached, we would like to use the serial port to relay the alarm condition to a global monitoring system. I don't know if the serial ports can accomplish this as the global monitoring system typically looks for a dry contact or a relay to be tripped. What are the capabilities of the serial port along these lines? Can I have it set to constantly send a 5V signal of dummy data and then drop that data transmission when the alarm condition is reached?

[Chris_Rake]

RS-232 can be +-12V or more depending on how the physical drivers are configured. However, if you can handle these voltages, then you can simply assert or deassert one of the modem control lines (RTS or DTR) to indicate your alarm state. If you want to use the TX line, you can set and clear the break condition. If you hook up a multimeter to one of these signals and experiment with NI-VISA in LabVIEW, you should be able to create a signal that will work in your system.

However, if you need +5/GND, you can use an RS-485 port which generates (well, in a way) signals between 0 and 5V (or 0 and 3.3V depending on the system). RS-485 is a differential signaling system, but you can just throw away one of the lines. In this case you'll use TX+, TX-, RTS+, or
RTS- depending on what you need.

[Tim.S]

mfino,

Were you successful integrating the suggestion by Mr. Rake? I have a similar problem (I want to trigger an audible alarm with the serial port) and I thought you might have some more information now.

Thanks